The specific aim of this proposal is to develop a prototype detection system for positron emission computed tomography (PCT) which will allow the attainment of the ultimate theoretically possible spatial resolution, and, as a consequence, marked qualitative and quantitative improvement in in vivo PCT imaging studies. In the proposed scheme several very narrow bismuth germanate (BGO) scintillator crystals are coupled to a large photomultiplier tube (PMT) (which provides the coincidence timing signal), and mercuric iodide (HgI2) photodetectors are coupled individually to each crystal (to provide a signal which identifies the crystal involved in a coincidence event). The proposed concept, which has only recently become possible as a result of advances in the development of mercuric iodide (HgI2) detectors and low-noise preamplification electronics capable of operating in conjunction with them at room temperature, circumvents the difficulties inherent in trying to design PCT systems with close-packed arrangements of narrow crystals coupled to PMTs of the size presently available. The first phase of the work will consist of maximizing the light output from the BGO crystals, designing and fabricating the HgI2 photodetector to optimize its response to the scintillation light, and finding the best way of optically coupling the BGO crystals to the HgI2 photodetectors and to the PMT. In the second phase of the work several prototype BGO-HgI2 detectors will be constructed and tested; the subsystem composed of the BGO crystals, HgI2 photodetectors, and large PMT will be designed, assembled and tested; the performance of the subsystem with its associated electronics will be evaluated in a tomography simulation system; and, based on the results of the project, a preliminary design of a complete PCT system will be made.